1. Field of the Invention
The present invention relates to a band-pass filter including a plurality of resonators formed on a dielectric plate, and a shared transmitting-and-receiving unit and a communication apparatus using the band-pass filter.
2. Description of the Related Art
One typical planar-circuit dielectric filter is a dielectric filter with attenuation poles at a low- or high-frequency region or both regions of the pass band, as disclosed in Japanese Unexamined Patent Application Publication No. 2000-13106, in which, for coupling resonators that are spaced at least one stage apart from each other, polarization coupling lines are formed on an input/output substrate or a cover which is a portion of a cavity, or otherwise, polarization coupling lines are formed on the upper and lower surfaces of a dielectric plate which is a filter substrate.
FIG. 19 illustrates the structure of a dielectric plate which is a typical filter substrate in the dielectric filter disclosed in the above publication. In FIG. 19, electrodes are formed over both surfaces of a rectangular dielectric plate. There are non-electrode portions 4a to 4e on the upper surface of the dielectric plate. There are also non-electrode portions having the same configuration as that of the non-electrode portions 4a to 4e formed on the lower surface of the dielectric plate so as to face the non-electrode portions 4a to 4e. The dielectric portions which are sandwiched between the non-electrode portions formed on the upper and lower surfaces of the dielectric plate serve as resonators. Accordingly, in the example shown in FIG. 19, the non-electrode portions or electrode-free portions 4a and 4e serve as input- and output-stage resonators, and the non-electrode portions or electrode-free portions 4b, 4c, and 4d serve as three resonator stages therebetween. A band-pass filter formed of a total of five resonator stages is thus constructed.
In order to produce an attenuation pole, a polarization line may be formed on a plate different from the dielectric plate shown in FIG. 19. This plate may be adjacent to the dielectric plate, and the second- and fourth-stage resonators may be magnetically cross-coupled, thereby producing an attenuation pole.
Meanwhile, as demand has increased for more compact, lightweight, and sophisticated electronic devices using such a planar-circuit dielectric filter, such as cellular telephones in particular, the dielectric filter is also required to be more compact and lightweight.
In the example shown in FIG. 19, the dielectric plate has an outer dimension of 18xc3x974.8 mm (86.4 mm2) where the relative dielectric constant of the dielectric plate is 24 and the center frequency of the pass band is 26.5 GHz. A need still exists for a more compact dielectric plate.
Furthermore, the number of stages of resonators must increase in order to achieve a sharp attenuation characteristic from the pass band to the stop band; this leads to a problem of increased size of the overall device.
A polarization coupling line which is formed in order to produce an attenuation pole may also lead to another problem of conductor loss due to the coupling line, resulting in low Q factor while increasing insertion loss. A separate provision of a substrate which carries a polarization coupling line may also lead to another problem in that any relative misalignment between this substrate and the dielectric plate which is a filter substrate would cause variations in the frequency of the attenuation pole to make the attenuation characteristic unstable, thereby requiring a strategy to overcome this problem.
Accordingly, it is an object of the present invention to provide a compact and lightweight band-pass filter which provides a satisfactory attenuation characteristic from the pass band to the stop band, and a shared transmitting-and-receiving unit and a communication apparatus using the band-pass filter.
To this end, in one aspect of the present invention, a band-pass filter comprising a dielectric filter includes electrodes formed on both upper and lower surfaces of a substantially rectangular dielectric plate, and a plurality of sets of substantially rectangular non-electrode portions which are adjacent to each other, each set of non-electrode portions facing across the dielectric plate, forming resonators in regions confined by the non-electrode portions on the dielectric plate. The resonators other than at least input- and output-stage resonators are nxcex/2 resonators, where xcex denotes one wavelength and n is an integer more than one, including a group of adjacent resonators which are capacitively coupled, and a group of adjacent resonators which are inductively coupled.
The direction in which the non-electrode portions are aligned differs depending upon whether resonators formed in the portions confined by non-electrode portions on the dielectric plate are capacitively or inductively coupled. The presence of a group of adjacent resonators which are capacitively coupled, and a group of adjacent resonators which are inductively coupled allows the non-electrode portions to be arranged, for example, in a staggered fashion rather than linearly, thereby reducing the rectangular dielectric plate in size in its longitudinal direction. The overall band-pass filter can be therefore more compact and lightweight.
The input-stage resonator may be inductively coupled with the resonator adjacent thereto, and the output-stage resonator may be inductively coupled with the resonator adjacent thereto. The resonators other than the input- and output-stage resonators may be capacitively coupled with each other.
Conversely, the input-stage resonator may be capacitively coupled with the resonator adjacent thereto, and the output-stage resonator may be capacitively coupled with the resonator adjacent thereto. The resonators other than the input- and output-stage resonators may be inductively coupled with each other.
With this structure, the direction in which the input- and output-stage resonators are aligned differs from the direction in which the remaining resonators are aligned, thereby reducing the dielectric plate in size in its longitudinal direction. This structure also provides cross-coupling every other resonator between the input- and output-stage resonators, and the resonators other than the input- and output-stage resonators which are coupled with each other, resulting in polarization.
The resonators other than the input- and output-stage resonators may be xcex resonators, where xcex denotes one wavelength, and may be arranged so that the longitudinal axes of the resonators are parallel to each other rather than linearly aligned. These resonators may be capacitively coupled with each other when d/L is greater than approximately 0.67, where L denotes the length of the resonators in the longitudinal direction, and d denotes the length of facing portions of adjacent resonators in the resonators.
Conversely, these resonators may be inductively coupled with each other when d/L is smaller than approximately 0.67.
Therefore, a band-pass filter can be constructed merely by defining a relationship between the length L of the resonators in the longitudinal direction and the length d of the facing portions of adjacent resonators, that is, with simplification in design.
In another aspect of the present invention, a band-pass filter comprising a dielectric filter includes electrodes formed on both upper and lower surfaces of a substantially rectangular dielectric plate, and a plurality of sets of substantially rectangular non-electrode portions, each set of non-electrode portions facing across the dielectric plate, forming resonators in regions confined by the non-electrode portions on the dielectric plate. The resonators are arranged so that the electric fields for the resonance mode used by the resonators are oriented in the same direction, and adjacent resonators in the resonators are shifted by a predetermined value in a parallel manner to the orientation of the magnetic fields.
Therefore, adjacent resonators can be electrically coupled, while resonators can be magnetically cross-coupled every other resonator, thereby achieving polarization.
In another aspect of the present invention, a band-pass filter comprising a dielectric filter includes electrodes formed on both upper and lower surfaces of a substantially rectangular dielectric plate, and a plurality of sets of substantially rectangular non-electrode portions, each set of non-electrode portions facing across the dielectric plate, forming resonators in regions confined by the non-electrode portions on the dielectric plate. The resonators are arranged so that the electric fields for the resonance mode used by the resonators are oriented in the same direction, adjacent resonators in the resonators are shifted by a predetermined value in a parallel manner to the orientation of the magnetic fields, and the longitudinal axes of the resonators are not parallel and at an angle with respect to the longitudinal and widthwise axes of the dielectric plate.
This structure allows the dielectric plate to be reduced in size in its widthwise direction.
In another aspect of the present invention, a band-pass filter comprising a dielectric filter includes electrodes formed on both upper and lower surfaces of a substantially rectangular dielectric plate, and a plurality of sets of non-electrode portions, each set of non-electrode portions facing across the dielectric plate, forming resonators in regions confined by the non-electrode portions on the dielectric plate. The resonators other than at least input- and output-stage resonators are dual-mode resonators which resonate in a mode for which an electric field is oriented in the direction of alignment of the resonators, and in a mode for which an electric field is oriented in the direction vertical (perpendicular) thereto, and adjacent dual-mode resonators are capacitively and inductively coupled with each other.
This allows a great number of stages of resonators to be formed on a restricted area of the dielectric plate, and coupling of dual-mode resonators allows for cross-coupling every two resonators.
In a further aspect of the present invention, a shared transmitting-and-receiving unit includes any of the above-described band-pass filters as a transmission filter and a reception filter. The shared transmitting-and-receiving unit can therefore be compact and lightweight.
In a still further aspect of the present invention, a communication apparatus includes any of the above-described band-pass filters or shared transmitting-and-receiving unit. The communication apparatus can therefore be compact and lightweight.
Other features and advantages of the present invention will become apparent from the following description of embodiments of the invention which refers to the accompanying drawings.